Our hypothesis is that each cortical representation of the visual field performs its own set of neural functions, some of which are unique to that particular area. One approach to determining the functions of cortical visual areas is the analyis of the response properties of single neurons to specific stimuli in a particular area in order to reveal how information is processed in that area. We propose to utilize this approach in our investigation of the functions of the third and fourth tiers of visual areas; however it has the definite limitaton that the animal is treated essentially as a passive object during the course of the experiment in which stimuli varying along physical parameters such as orientation or velocity are presented and the responses of single neurons to these different physical parameters are recorded and compared with responses to other parameters. None of this takes into account the ongoing perceptions of the monkey. We have conceived a second approach in which we are asking the question: At what level in the visual system do the responses of neurons co-vary with the consciously perceived stimulus as opposed to merely the physical stimulus? Often the brain is able to construct more than one interpretation of the total set of incoming visual information so that there exist rivaling perceptions of the visual scene. One rather extreme type of perceptual rivalry occurs when each eye independently views a different scene. This is binocular rivalry and results in perception alternating from what the one eye sees to what the other eye sees. We propose to record from single neurons in the visual system of monkeys while they are experiencing binocular retinal rivalry and reporting to us their ongoing changes in perception in order to determine at what level in the visual system respond differently according to whether or nor the stimulus is consciously perceived.